She is turning 12 next weekend and she really loves Minecraft. For her last bday I bought MC for her. She sort of mentioned that she'd like to have a MC party but I'm pretty bad at planning parties, and even worse at planning themed parties. Any suggestions?

THIS is what someone else did for their 9yr old's birthday, Cheap and easy things.

I love the pinata idea, maybe candy, because everyone loves candy in a slime theme box. (creeper head, or a ghast?)

the paper taped to the walls for trees is always a nice touch. Print off the texture from the papercraft site for better visual appeal.

"Crafting table" made from electrical tape on the kitchen table is also good idea.

Other suggestions:

find a big cardboard box and cut it so it opens like a chest. Hide presents inside it.

Have a skin crafting party making Steve head masks, Pig masks, Creeper masks, etc.

Have a tool crafting party, Hide some blocks (iron, gold, and diamond) around the house, who ever finds the block keeps it to trade them for pre-made Gold swords, diamond swords, etc. (You could also give out goodies for finding the rarer blocks as well)

Cake: a minecraft cake, a diamond cake, or a mob cake. I believe you can get edible paper transferred onto a cake. Something like this If you dont want to craft one yourself. I think most grocery stores with a bakery will do this for you as well.

Pin the tail on the donkey knock off, Do a "Kill the mob with an arrow".
arrow could be a wooden skewer with a paper arrow tail. and the mob could be on top of cork board. I would suggest not to throw the arrow, liability could be high risk. (not sure how well this one would work)

Balloons that can be drawn on to look like glowstone, and let them float to the ceiling. (or hide them in papercrafted glowstone blocks) Or just computer paper to make square clouds.

get green table cloths to throw over extra tables. Make tiny grass blocks on top for center pieces, Get some red flowers too.

I suppose a good start would be to have a quick in-game party with her and some of her friends if they have the game. I would also suggest this site for printing out assorted blocks/characters for decoration around the house. Depends what your child is into to be honest. :)